1. Field of the Invention
The present invention relates to a light guide plate for a light source of sidelight type, a surface light source including the light guide plate, and a liquid crystal display device including the surface light source.
2. Description of the Related Art
FIGS. 9A and 9B are plan views schematically illustrating a conventional light source of sidelight type. As shown in FIG. 9A, a light guide plate 108 has an end surface provided with a plurality of LEDs (Light Emitting Diodes) 106. An example of a method of reducing unevenness in luminance of the surface light source with an aim to improve the luminance includes a method of mounting the LEDs 106 with high density. Specifically, a mounting interval between the LEDs 106 adjacent to each other is reduced from a mounting interval B of FIG. 9A to a mounting interval C of FIG. 9B, which increases the mounted number of LEDs 106. The mounting interval can be defined as a distance between adjacent light sources in the periphery direction of the light guide plate.
However, shortening the mounting interval C is subjected to restrictions specific to the shape of the LEDs 106 or a device on which the LEDs are to be mounted. Moreover, if the mounting interval C is short, heat generated by the LEDs 106 adjacent to each other reduces light emitting efficiency. Generally, to prevent the thermal destruction of the LEDs 106 caused by a temperature rise, the maximum current for driving the LEDs 106 to emit light is restricted. If the LEDs 106 adjacent to each other suffer from reciprocal interference of heat generated by themselves, a stricter restriction is put on the maximum current. As a result, only a reduced current can be applied to the LEDs 106. Therefore, it is difficult to allow the LEDs 106 to brightly emit light.
Japanese Laid-Open Patent Publication No. 2004-79488 (especially see FIG. 4) discloses that LED elements are arranged on light entrance planes formed on two facing edges, wherein the LED elements are arranged such that optical axes do not overlap with each other.
Moreover, Japanese Laid-Open Patent Publication No. 2003-109421 (especially see FIG. 1) discloses that a plurality of light entrance planes are provided with a plurality of light sources arranged such that light incident angles are different from each other.
In Japanese Laid-Open Patent Publication No. 2004-79488, the LED elements are arranged on the two facing edges, which causes a problem that a frame region of a liquid crystal display device increases. Moreover, in Japanese Laid-Open Patent Publication No. 2003-109421, the light sources are arranged on a backside of a light guide plate, which causes a problem that the thickness of a backlight unit increases.